In long-haul optical networks, nonlinear optical effects can degrade the optical signal transmission. Such nonlinear optical effects include four-wave mixing (FWM) and Cross-Phase Modulation (XPM). Introducing dispersion in an optical fiber can reduce such nonlinear effects.
The dispersion product or accumulated dispersion of a span of fiber is a measure of the dispersion accumulated over the span. The dispersion product for a fiber of length L with a dispersion D is the product of L and D, i.e., L•D. Thus, the dispersion product of a span of fiber having individual sections of length Li and dispersion Di is the sum of the individual dispersion products ΣLi•Di. Dispersion units are typically given as picoseconds/nanometer-kilometer (ps/nm-km), where the kilometer units correspond to the length of the fiber.
Optical transmission systems must periodically compensate for the accumulated dispersion. Compensation is typically accomplished by periodically inserting a section of dispersion compensating fiber (DCF) between the sections of transmission fiber to reduce the accumulated dispersion.
A scheme that compensates for accumulated dispersion should also compensate for the dispersion slope. Many optical transmission systems operate according to transmission schemes, such as wavelength division multiplexing (WDM), using multiple transmission wavelengths to increase the overall bandwidth of the transmission. The system transmits over a band of several channels, each channel transmitted at a different wavelength within a range of transmission wavelengths. In general, dispersion is wavelength dependent and will be different for each of the channels. Thus, any reasonable scheme for compensating dispersion in a multiple wavelength transmission scheme should take care to compensate the dispersion slope, S, i.e., the change in dispersion per unit wavelength. The units of dispersion slope are typically given as picoseconds/(nanometer)2-kilometer (ps/nm2-km).
To compensate for both dispersion DT and dispersion slope ST of the transmission fiber certain conditions must be satisfied. To understand the requirements for dispersion DDC and dispersion slope SDC of a DCF, it is usefull to consider a typical terrestrial transmission system with a length LT of transmission fiber and a length LDC of DCF between optical amplifiers. The transmission system transmits over multiple channels with a center channel having a center wavelength λC. In this case, the dispersion DT of the transmission fiber must be compensated by the DCF. Accordingly, the following condition must be satisfied, DDC(λC)•LDC=−DT(λC)•LT. Additionally, the dispersion slope ST of the transmission fiber must be compensated by the dispersion slope SDC of the DCF. Accordingly, a second condition must be also satisfied, κDC(λC)=(DDC/SDC)=κT(λC)=(DT/ST).
One transmission fiber finding increased use in optical transmission systems is LEAF (large effective area fiber). LEAF typically has a κ value of approximately 50. It has been difficult to design a DCF to compensate for the low κ value of a LEAF. A number of schemes have been put forth to achieve compensation of LEAF fibers. For example, higher order dispersion compensating devices, dispersion compensating gratings, and photonic crystal fiber have all been proposed. Each of these solutions has problems and faces different technological issues. In addition, the use of dual fiber dispersion compensation has also been proposed. In dual fiber dispersion compensation, the dispersion and dispersion slope are compensated in separate fibers, i.e., dual fibers. However, dual fiber dispersion compensation has not found a lot of acceptance to date in part due to the difficulty in designing the dispersion slope compensating fiber.